Blow off – BOV

We are all familiar with the characteristic blowing and whistling of turbocharged engines, mostly in modified cars. This sound effect is created by the blow-off (BOV) bypass valve.

The role of the BOV valve:

The purpose of the blow-off valve is to release the accumulated excess pressure that builds up in the intake tract between the turbocharger (or compressor) and the throttle body.  At the very moment when the throttle valve closes (e.g. when shifting gears) the turbo is still pushing air towards the closed throttle valve by its inertia. This air then bounces away from the throttle towards the turbo, strikes the turbo's blades, slows its rotation, creates excess pressure and with increasing resistance the turbocharger's heat load also increases.

The blow-off valve releases this excess pressure, the turbo does not lose its speed and thus the so-called turbo hole is eliminated when shifting. Of course, the BOV valve does not increase the engine's power, but it does help the turbo spool up faster and the turbocharger's potential can be better exploited. It also has a positive effect on the life of the turbo blades and bearings as it reduces the load on them.

BOV valve principle:

The principle of operation of the BOV valve is quite simple. Basically, it is a diaphragm or piston with a spring, located before the throttle and after the turbo. It is connected to the manifold (behind the throttle) by a vacuum hose. When the throttle is closed, this compressed air in front of the throttle and the vacuum behind the throttle, opens this valve and releases the excess pressure. Depending on where this accumulated pressure is released, we divide BOV valves into three categories.


- This valve releases excess air into the surrounding atmosphere, causing the popular sound effect

- There are also sequential double diaphragm valves, where only one smaller diaphragm opens at lower pressure and both diaphragms open at the same time at higher pressure


- This BOV valve returns compressed air back to the intake before the turbocharger

- It does not create a sound effect, for this feature it is often used by automakers because of the noise emissions


- These BOV valves have a double outlet to both intake and atmosphere

BOV valve selection

When selecting a BOV valve, it is important to decide, in addition to the design, whether you want to use a universal , or choose a valve to suit your vehicle.

Universal valves have the advantage of a wide choice and lower price, but with a universal valve you also have to take into account various other modifications of the intake tract.

If you do not want to interfere with the intake design on your engine, or if you have a vehicle with more sensitive intake management (modern engines) we recommend a plug and play valve, directly designed for the specific engine type. Although these valves are slightly more expensive , installation is simple and replacing the stock valve will often solve some ECU problems such as engine knock in some modes.

We offer a range of BOV valves from leading manufacturers such as HKS, TURBOSMART or NUKE Performance, or even unbranded cheaper alternatives.

You can also find universal BOV valve kits for diesel engines in our range. However, with these, due to the principle of diesel engine operation, none of the above advantages apply, it is just an added sound effect :)

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